than traditional EFIE, respectively. This justiﬁes again the use of the shift scheme from the principle value term of MFIE oper- ator is effective for constructing SSOR preconditioners in IEFIE. 5. CONCLUSIONS AND COMMENTS In this article, a novel shift scheme is presented and used for constructing robust SSOR preconditioners for efﬁciently solving IEFIE formulation of electromagnetic scattering problems. It leads to a signiﬁcant reduction in the solution time while achiev- ing reasonable accuracy of ﬁnal result. Numerical experiments on two examples are performed and comparison with EFIE, IEFIE, and traditional SSOR preconditioned IEFIE is made, which shows the new preconditioner is more efﬁcient and ro- bust. Furthermore, Numerical results also demonstrated the novel preconditioner will not be failure, when combination fac- tor or the number of updating steps in IEFIE changes. This fea- ture means effect of this shifted strategy is quite independent and stable. REFERENCES 1. S.M. 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TABLE 1 Comparison of Solution Time (in seconds) with EFIE, IEFIE, IEFIE Imposed SSOR, and IEFIE Imposed S-SSOR on Two Examples Geometry Solution Time (s) EFIE IEFIE IEFIE þ SSOR IEFIE þ S-SSOR(3) Multiobject structures 55.09 30.41 27.83 13.36 Open cavity 3788.59 781.23 1472.52 450.27 A ROTATED ELLIPTICAL SLOT ANTENNA WITH ULTRA-WIDE BANDWIDTH Jia Lao, Ronghong Jin, and Junping Geng Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai, China; Corresponding author: James_lao@msn.com Received 14 May 2012 ABSTRACT: A novel ultra-wideband elliptical slot antenna is proposed. An elliptical slot is rotated by 45  and excited by a circular patch. Tapered microstrip feeding is used for impedance matching. The structure has multiresonance feature over a wide frequency range, where the simulated bandwidth ratio exceeds 20:1. Due to the limitation of the SMA connector, the experiment on the prototype antenna demonstrated an impedance bandwidth (voltage standing wave ratio < 2) of 165%, or 2.54–26.6 GHZ, 10.5:1. V C 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:308–310, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27288 Key words: slot antenna; rotated; ultra-wideband 1. INTRODUCTION In the emerging ultra-wideband (UWB) communication systems, antennas with large bandwidth and simple structures are gener- ally desired. Comparing to traditional Vivaldi, spiral, and log- periodic wideband antennas, numerous printed slot antennas fea- turing low proﬁle, light-weight, and low fabrication cost have been reported over the years, especially the wide slot antennas with nice impedance characteristics. The feeding structure could be either microstrip or CPW [1]. Various kinds of tuning stubs have been studied to improve the bandwidth [2, 3], as well as different slot shapes [4, 5]. A simple rotated square slot antenna with straight feed line has also been studied, which introduced new resonant modes and quadrupled the bandwidth [6]. In this article, a novel printed antenna with rotated elliptical slot is proposed and investigated. The antenna is excited by a feeding structure consisting of a circular patch and a tapered feed line. By rotating the slot, the antenna achieves multireso- nant characteristics over a very large frequency span. Simulation showed an extreme bandwidth ratio of over 20:1 at voltage standing wave ratio (VSWR < 1.7), whereas limited by the con- nector, the experiment veriﬁed a bandwidth of 2.54–26.6 for VSWR < 2, or bandwidth ratio of 10.5:1. 2. ANTENNA GEOMETRY In this design, we combined several bandwidth expansion techni- ques. First, comparing to rectangular and circular slots, elliptical Figure 1 Geometry of the antenna 308 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 2, February 2013 DOI 10.1002/mop